1. Field of the Invention
The present invention relates to a process and apparatus for the separation of a polar fluid from a non-polar fluid using a fluid separation membrane.
2. Related Art
There is a variety of fluids that include polar and non-polar components in which it is desirable to separate the polar component from the non-polar component with the use of membranes, whether the membrane is a gas separation membranes or a pervaporative membrane.
One type of fluid including both polar and non-polar components is flue gas. Depending upon the type of oxidant used to combust the fuel and produce the flue gas, the flue gas can contain anywhere from 4-80% CO2 along with minor yet sometimes significant amounts of SO2 and NO pollutants. By NOR, we mean one or more of oxides of nitrogen, including NO, N2O, N2O4, and NO2 and N2O3. While much of the SO2 and NO in the flue gas is removed from the flue gas by a variety of conventional pollutant treatment methods, some CO2 applications require even lower amounts of these pollutants. In recognition of this, some have proposed the use of a De-NOx column which separates the feed fluid into a CO2 enriched gas for further purification in a stripping column and a NOx-enriched liquid. However, the use of a De-NOx column still presents the challenge of dealing with the NO2-enriched liquid bottom from the column. For example, U.S. Pat. No. 7,708,804 proposes three solutions: 1) vaporizing and recycling the NO2-enriched liquid to the inlet of the compressor, 2) feeding the NO2-enriched liquid to a wash column, and 3) combustion of the NO2-enriched liquid in the flame of a burner.
While the approaches disclosed in U.S. Pat. No. 7,708,804 do present reasonable solutions for removal of the NO from the CO2, each approach exhibits a disadvantage. Because the recycle stream in the first technique may represent about 5-10% of the total flow compressed and treated downstream of the compressor, the compressor and downstream equipment must be sized 5-10% larger than it would have to be if the NO2-enriched stream was otherwise not recycled. There would also be a 5% to 10% increase in the required compression energy. Furthermore, the relatively higher acid gas content of the flue gas being compressed will produce a greater amount of acid gas condensate in the compressors, subjecting the compressors and driers to a more severe acidic attack in comparison to the absence of a NO2 recycle stream. This more acidic attack may lead to a decreased useful lifetime for the compressors or require the compressor to be constructed of a more costly material that is sufficiently resistant to such acid fluids. Similar negative impacts upon the driers would be expected to occur due to the presence of the acid gas. The use of wash column would result in significant CO2 losses unless the post-wash column fluid was recycled to the compressor. Reduction of NO2 in the flame of a burner still results in significant CO2 losses.
Thus, it is an object to propose an improved process and apparatus for separating NO2 from a liquid containing CO2 and NO2 that does not exhibit the above-mentioned disadvantages.
Another type of fluid including both polar and non-polar components is natural gas. In order to avoid corrosion of natural gas pipelines, the CO2 and H2S in the raw natural gas need to be removed. The current state of the art technique for the removal is the use of amine based adsorption systems to separate these gases. However this technology is highly energy intensive and is not environmentally friendly.
Thus, it is another object to propose an improved process and apparatus for separating H2S and/or CO2 from natural gas that does not exhibit the above-mentioned disadvantages.
Membrane separations on the other hand are less energy intensive and environmentally friendly. While commercially available membranes such as cellulose acetate- and polyimide-based membranes can be used for this separation, there is a desire for commercial use membranes that have relatively greater fluxes.
Two terms, “permeability” and “selectivity”, are used to describe the most important properties of membranes:productivity and separation efficiency respectively. Permeability (P) equals the pressure and thickness normalized flux, as shown in the following equation:
                              P          i                =                                            n              i                        ·            I                                Δ            ⁢                                                  ⁢                          p              i                                                          (        1        )            where ni is the penetrant flux through the membrane of thickness (I) under a partial pressure (Δpi). The most frequently used unit for permeability, Barrer, is defined as below:
                    Barrer        =                              10                          -              10                                ⁢                                                    cc                ⁡                                  (                                      S                    ⁢                                                                                  ⁢                    T                    ⁢                                                                                  ⁢                    P                                    )                                            ·              cm                                                      cm                2                            ·              s              ·              cmHg                                                          (        2        )            Selectivity is a measure of the ability of one gas to flow through the membrane over that of another gas. When the downstream pressure is negligible, the ideal selectivity (based upon the permeabilities of pure gases) of the membrane, can be used to approximate the real selectivity (based upon the permeabilities of the gases in a gas mixture). In this case, the selectivity (αA/B) is the permeability of a first gas A divided by the permeability of a second gas B.
Polyimide based hollow fiber membranes are currently used for the separation of H2S and CO2 from natural gas. These membranes are made by spinning porous polyimide membranes and post-treating them with sylgard, a poly (dimethyl-siloxane) polymer. The resulting membrane has properties of both the base polyimide polymer as well as sylgard. Since sylgard is a non-polar material, it has less affinity towards CO2 and exhibits an inferior CO2/CH4 separation. This non-polar characteristic of sylgard compromises the net separation characteristics of composite membrane.
Thus, it is an object to provide a membrane for the separation of H2S and CO2 from natural gas that exhibits a sufficiently desirable selectivity for these gases over methane.
The advantages of combining the high permeability of siloxane polymers such as polydimethyl silicone (PDMS) and the high affinity for polar species of polyethers such as polyethylene glycol (PEG) have been long recognized. For example, U.S. Pat. No. 4,606,740 and U.S. Pat. No. 4,608,060 prepared membranes by blending PDMS+PEG and showed increased selectivity and high permeance for polar gases such as CO2, SO2 and NH3. However these blended materials are mechanically weak.
Thus, it is an object to provide a membrane material for the separation of polar fluids from non-polar fluids that is sufficiently mechanically strong.
Park, et al. studied polyurethanes and polyurethane ureas forming hard/soft segmented copolymers by reacting a diisocyanate (MDI) with various polyalkyl oxides and PDMS (Park, et al., J Membrane Science, vol. 204, pp 257-269, 2002). In these polymers, the individual PDMS and polyether moieties are separated by MDI linkages. The material CO2/N2 permeability-selectivity does not appear to be commercially viable.
Thus, it is an object to provide a process and apparatus for the membrane separation of polar fluids from non-polar fluids that is commercially viable.
Reijerkerk, et al. studied blends of a copolymer of 20% PDMS-80% PEG with PEBAX block copolymers (Reijerker, et al., J Membrane Science, vol. 352, pp 126-135, 2010). High polar gas permeability-selectivity can be obtained at high ratios (>50%) of the PDMS-PEG copolymer additive. However, these blend materials are expected to have the same weakness as the U.S. Pat. No. 4,606,740 approach.
Thus, it is an object to provide a process and apparatus for the membrane separation of polar fluids from non-polar fluids that is not mechanically weak.
Polyethers are known to have high affinity for polar gases (Lin & Freeman, J Membrane Science, vol. 239, pp 105-117, 2004). Also, several patents describe membrane materials based on a hard-soft segmented copolymer where the hard (rigid) segment could be a polyamide, polyester or polyurethane while the soft (flexible) segment is the polyether. Examples of such approaches include U.S. Pat. No. 4,963,165, U.S. Pat. No. 6,843,829 and U.S. Pat. No. 6,860,920. PEBAX is an example of a commercially available hard-soft segmented copolymer where the hard segment is a polyamide.
Therefore and in view of the disadvantages of conventional processes and apparatuses, it is an object of the invention to provide a membrane-based process and apparatus that does not exhibit those disadvantages.